The invention concerns a new coating for paper and other substrates for use in direct thermal printing.
Thermal printers selectively heat a heat-sensitive coating to print on a suitable substrate stock such as paper or paperboard. They can be used for a wide variety of purposes, most of which call for sharp contrast between white and dark areas and uniform density of the dark areas even with varying energy inputs.
Direct thermal print media typically comprise three layers coated on paper or other substrate: a first coating, in contact with the substrate of clay and a binder; a second, active layer comprising the thermally sensitive components; and, a top coat of a material suitable to protect the active layer before and after image formation. The active layer typically comprises a dye, a sensitizer and a color developer (also sometimes referred to as a coreactant).
A number of prior patents list various combinations of active layer ingredients.
In U.S. Pat. No. 5,066,634, Minami, et al., describe heat-sensitive recording materials which are said to have superior heat, water and oil resistance. To achieve this they employ a specific developer of 4-hydroxy-4xe2x80x2-n-propoxydiphenylsulfone and at least one fluorane-leuco dye from a specific class. The data show comparable image densities for the invention and the comparisons, with differences in the noted properties of heat, water and oil resistance in the absence of an overcoating layer.
In U.S. Pat. No. 5,116,802, Arai, et al., describe a heat sensitive recording material which, in the absence of an overcoating layer, will avoid deposits on the thermal print head. They disclose the combination of di(p-methylbenzyl) oxalate with a basic dye comprising 3-di(n-butyl)amino-6-methyl-7-phenylaminofluoran and a developer such as 4-hydroxy-4xe2x80x2-isopropoxydiphenylsulfone or 4,4xe2x80x2-isopropylidenediphenol.
There is a need for thermally sensitive recording media having high contrast when printed and comprising an overcoating designed for long life of the printed image.
The invention provides both a new print stock specially adapted for use in thermal printing and to a composition and a method for making it.
A coating composition is applied to a print stock substrate for preparing a thermally-active layer, the coating composition comprising:
a dye comprising at least one or a combination of 6xe2x80x2-dipentylamino-3xe2x80x2-methyl-2xe2x80x2(phenylamino)-spiro [isobenzofuran-1(3H),9xe2x80x2-[9H]xanthen]-3-one (CAS No. 129473-78-5) and 6xe2x80x2-dibutylamino-3xe2x80x2-methyl-2xe2x80x2(phenylamino)-spiro [isobenzofuran-1(3H),9xe2x80x2-[9H]xanthen]-3-one (CAS No. 89331-94-2);
a sensitizer comprising at least one or a combination of 1,1xe2x80x2-sulfonylbisbenzene (CAS No. 127-63-9) and dimethylterephthalate (CAS No. 120-61-6); and
a coreactant (or developer) comprising at least one or a combination of bis(4-hydroxyphenyl)-4-methylpentane (CAS No. 6807-17-6) and 4-hydroxy-4xe2x80x2-isopropoxydiphenylsulfone (CAS No. 95235-30-6).
Also preferably-the thermally-active layer will be overcoated with a protective layer.
The method of the invention comprises preparing the above composition and applying it to a print stock substrate. The product of this process is also new.
Many of the preferred aspects of the invention are described below.